1. Field of the Invention
The present invention is directed to a process and apparatus for cleaning filters. More specifically, the present invention is directed to a process and apparatus for cleaning filters by contacting the filters to be recycled or disposed of with liquid or supercritical carbon dioxide.
2. Background of the Prior Art
At present when a filter, for whatever purpose, is no longer effective because its pores are filled with the material to be filtered from a fluid stream, such filters are xe2x80x9cthrown away.xe2x80x9d This practice leads to environmentally unfriendly results. It is not always known what agents contaminate the used filters. Such agents are oftentimes harmful to the environment even if the used filter is buried or combusted.
Not only does the present practice of disposing of used filters present an environmental problem but the replacement of many of these filters represent a high economic cost. Indeed, certain filters are very expensive and thus their replacement exacts a high economic price.
To elaborate this latter point, filters formed of polyfluoroethylene and other expensive polymeric materials employed in processing steps involved in the formation of semiconductors, food products and the like can each cost as much as $1,000 or even more.
The cost involved in discarding plugged filters, furthermore, results in the discarding of a filter core. The filter core merely serves to support the filter element and facilitate drainage rather than removing particulates, which, or course, is the function of the filter. Thus, even if the filter cannot be cleaned and recycled, it is apparent that the disposal of filter cores represents an unnecessary economic and environmental waste.
In recognization of the economic and environmental savings to be obtained by retention of filter cores, attempts have been made to design filter apparatus in which only the filter element is replaced. The purpose of such designs is to retain cores so as to eliminate the environmental and economic losses associated with their replacement. In such designs, the replaceable filter element is slipped onto a support core and clamped to the core with rings or other compression-based appliances. Unfortunately, the application of such appliances to a filter element compresses the filter medium and results in disruption of uniform filtering characteristics and flow. As a result, structural integrity and filtration performance of the filter element may be impaired. Furthermore, unless the replacement of the filter element is done carefully, the filter element can be damaged. As such, a design which permits separation of the core and the filter element, to overcome the environmental and economic costs associated with filter replacement, is not totally availing.
Another problem associated with conventional reusable core type filter assemblies is the absence of any valve-type structures for pressure relief, temperature sensitive flow control and filter by-pass. Rather, filter assemblies provided with valve-type structures, essential in continuous fluid flow applications where circulation is paramount, are limited to unitary filter assemblies where the core is not reusable.
Among filters which meet the parameters discussed above, i.e. high cost filters having very small pore size, are filters employed in photoresist and other semiconductor processing. U.S. Pat. Nos. 5,554,414; 5,698,281; 5,885,446 and 6,000,558 all describe filters which are useful in these applications. Such filters are formed of woven nylon, polypropylene and polytetrafluoroethylene membranes or film cartridges. Presently, such filters are disposed of when they become plugged.
The reason for the inability, in the prior art, to overcome these problems by cleaning filters is that fluids usually employed in cleaning such polymeric surface are aqueous or organic solvents having physical properties which cannot penetrate very small pores. As indicated in standard texts, such as Adamson, xe2x80x9cPhysical Chemistry of Surfaces,xe2x80x9d Chapters 1 and 7, J.Wiley (1976), an effective cleaning fluid must wet the surface and have a low contact angle. The surfaces of polymeric surfaces employed in filters, such as the surface of polytetrafluoroethylene, often have low surface engergies, i.e. in the range of about 10 to 20 dynes/cm. However, water and acetone, typical solvents used in such operations, have surface energies of about 80 dynes/cm and 30 dynes/cm, respectively. As such, these typical cleaning solvents do not wet such polymeric surfaces. Therefore, high back pressure is required to backwash a clogged filter to overcome the viscous drag forces of backwashing.
In addition to the problems associated with the high surface energy of fluids typically employed in cleaning debris from filters, an additional factor that discourages their use is the requirement that a still further drying step is required, subsequent to any successful utilization of such fluids in cleaning filters, given the fact that the aqueous or organic fluid employed in cleaning filters is not completely removed in the filter cleaning operation.
The above remarks establish the need in the art for an effective cleaning method to remove contaminants from filters having very small pore sizes.
A new process has now been developed which permits the cleaning of filters of all types including those which having nanometer-sized pores. This new process is effective insofar as it permits the cleaning fluid to wet the polymer filter surface. The effectiveness of this new process is due to the novel physical properties of the cleaning fluid.
In accordance with the present invention a process of cleaning filters whose pores are fouled with particulates is provided. In this process a particulate-fouled filter medium is contacted with liquid or supercritical carbon dioxide composition. In further accordance with the present invention an apparatus for cleaning filters is provided. The apparatus of the present invention includes means of contacting a particulate-fouled filter medium with a liquid or a supercritical carbon dioxide composition.